A baculovirus expression system was used to express different isoforms of calcineurin by infecting permissive insect cells. Calcineurin is a serine-threonine phosphatase modulated by calcium and calmodulin. The enzyme is composed of two subunits, a catalytic subunit CNA (60 Kda) and a regulatory subunit CNB (20 Kda) with some homology with calmodulin. We observed recently that the co-infection of insect cells with recombinant baculoviruses selected to conduct the expression of the two subunits of calcineurin induced the formation of cytoplasmic corpuscles very refringent in the infected cells. These "crystalloides" appeared within an optimum time between 48h and 72h after infection and were present only when the two subunits of the enzyme were produced. The "crystalloides" were regular in shape and look like bi-pyramidal, tetragonal with square or losangic sections. The number and size differed from cell to cell but measured up to 10 um, as estimated by standard light microscopy methods. We wished to know whether these corpuscules are real organized crystals of proteins or represent an aggregation form (inclusion body) or denatured proteins. BM study revealed that these corpuscles are protein crystals with spacing of -5.5 nm. IvEM was essential in this study in the determination of the symmetry of the crystalloids since we needed to cut 1-2 um sections to find crystalloids that were completely enclosed within the section. We also studied the 3D morphology of the infection process in the nucleus. A manuscript is in press (Fan et al., 1996, Microsc. Res. Tech.).